i40e: stop flow director on shutdown

In some cases, the hardware would continue to try to access the FDIR
ring after entering D3Hot state, which would cause either PCIe errors or
NMIs, depending upon system configuration.

Explicitly stop FDIR in our shutdown routine to eliminate this
possibility.

Change-ID: Ib98060d6352ec595ab9a78bfe252675a9fa5d8bc
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>